The invention relates to a process for the preparation of secondary monomethylalkyl amines by reduction of a hexahydrotriazine compound in the presence of a metal component.
Such a process is known from U.S. Pat. No. 2,657,237 in which a process is given for the continuous preparation of dimethylamine by dissociation of trimethyltrimethylene triamine into monomeric methylmethylene amine, and subsequent catalytic hydrogenation of the monomer into dimethylamine. The process taught is complicated in that it requires specific engineering and equipment to enable the continuous process to be carried out, and a mixture of hydrogen and the liquid trimethyltrimethylene triamine has to be vaporized in a separate preparatory step. Thus, the process is carried out by many sequential process steps, including the separate vaporization step, reaction of the components, separation of the reaction product and feeding the reaction product to a second reaction means. In the second reaction means gaseous reactants are to be passed through a catalyst, upon which dimethylamine is formed. Another drawback of such a process is that its use is uneconomical for the production of monomethyl amines having large alkyl groups, since vaporization of the precursors is rather complicated by their lower volatility.
"Bequeme Darstellung von reinen N-Methylalkylaminen durch Zink/Salzsaure-Reduktion von 1,3,5-Tris(alkyl)-hexahydro-1,3,5-triazinen", M. Al Schaik and H. Oelschlger, Arch. Pharm. 317, pp. 214-219 (1984) describes a process for preparation of n-methyl alkyl amines by zinc/hydrochloric acid reduction of trialkylhexahydrotriazines. The process is carried out at -5.degree. C. using a drop-in procedure which is rather uneconomical. Further, the use of a chemical type of reduction is generally considered a major drawback. Together with the use of highly corrosive hydrochloric acid, the method described is expensive and difficult to practice on plant scale. Also the amine is obtained as its hydrochloric acid salt, from which it must be liberated with, for example, sodium hydroxide. Another drawback of this procedure is the large amount of zinc chloride waste produced and the rather low yield of the desired product obtained.
The Netherlands patent NL 7008733 relates to a method for the preparation of secondary or tertiary amines. In this patent specification, secondary or tertiary amines are prepared by reaction of a primary amine with formaldehyde in the presence of H.sub.2, an alcohol and a catalyst. By this reaction, no hexahydrotriazine as used according to the present invention is formed, nor is a highly selective process for the production of secondary monomethylalkyl amines given.